A large number of internal voltages of different magnitude are required in integrated semiconductor circuits, for example, in dynamic semiconductor memory modules DRAM, in order to supply internal functional units and to operate them correctly. The output voltage is as constant as possible and is provided with adequate current driver capability, with as low an impedance as possible.
As is known, a DRAM includes memory cells with a storage capacitor, whose state of charge represents the stored information. Due to leakage currents, the stored state of charge in the capacitor is changed, and the separation from a reference decreases. In order to make it possible to read the stored information without errors in spite of this, it is necessary for the reference levels used to be as constant as possible and to maintain a predetermined level of magnitude, even in poor operating states. For example, a voltage generator, which is located centrally between the voltage levels that represent the two binary logic states, is required. Since the information to be read is compared with this central voltage level, its accuracy is subject to relatively stringent requirements. Finally, further potentials which supply the memory cell array and the circuits for reading and writing are also provided by a higher-level voltage generator arrangement.
Such a voltage generator arrangement includes two or more stages. A bandgap reference circuit provides an output potential, which is referred to as reference ground potential, and is largely independent of external operating influences, such as the external supply voltage or temperature. The bandgap reference circuit has a high-impedance output. The bandgap reference circuit is thus expediently followed on the output side by an impedance converter, which transforms the reference potential, that is provided with a high impedance, to a low impedance. Finally, the impedance converter drives a voltage generator, which is arranged on the output side and supplies an output potential that is as constant as possible and has a high current driver capability, and whose magnitude is set as a function of the output signal from the impedance converter. Two or more impedance converters may be driven in parallel by the same bandgap reference circuit, or various output-side voltage generators may be provided in order to produce different output voltages, or the same voltages which can be fed in at different points on the semiconductor chip.
For such a voltage generator arrangement separate reference ground potential lines have been provided. The bandgap reference circuit and the impedance converter are connected to a first reference ground potential line. The bandgap reference circuit and the impedance converter draw a constant current irrespective of the various operating states of the DRAM. Furthermore, the current that is drawn is relatively small. The voltage drop along this line is thus constant, or can easily be compensated for. The output-side voltage generator is connected to a second reference ground potential line, which is separate from the first. The two reference ground potential lines are, for example, formed from metal tracks, which run in a metallization plane on the semiconductor chip and which, for example, are composed of aluminum or of an aluminum alloy. The reference ground potential is supplied from the exterior via what is referred to as a connecting pad.
Various pads are also feasible, which are then connected to one another externally to the chip. The two reference ground potential lines which have been mentioned are typically connected via the connecting pad at least to the external supply for the reference ground potential.
Since the current, which is not inconsiderable during operation, is supplied via the external voltage generator to a load that is to be driven, and this current flows back via the second reference ground potential line to the connecting pad, in which case the current that is drawn can also fluctuate relatively severely as a function of the operating states of the DRAM, the voltage drop along the second reference ground potential line is no longer negligible. A voltage drop is thus produced between the connecting pad and that point at which the output-side voltage generator makes contact with the second reference ground potential line. This voltage drop can fluctuate over time.
The described voltage generator arrangement is subject to the problem that the reference generator and the impedance converter are always supplied with a constant reference ground potential, while the potential at the reference ground potential connection for the output-side voltage generator fluctuates as a function of the current flowing via the second reference ground potential line. Thus, during operation, the reference ground potentials for the output-side voltage generator on the one hand and for the bandgap reference circuit and impedance converter on the other hand differ from one another.
In particular, as the miniaturization of the structures on the integrated semiconductor chip progresses and as the complexity of the circuits to be supplied increases, there is a trend on the one hand to reduce the internal voltages further although, on the other hand, higher currents are required, even though the resistances of the metallization lines increase. As a consequence of these boundary conditions, it is problematic to provide the required internal voltages with sufficient constancy and a sufficiently high current drive capability with the use of conventional concepts.